Thin‐filament regulation of force redevelopment kinetics in rabbit skeletal muscle fibres

Thin-filament regulation of isometric force redevelopment (ktr) was examined in rabbit psoas fibres by substituting native TnC with either cardiac TnC (cTnC), a site I-inactive skeletal TnC mutant (xsTnC), or mixtures of native purified skeletal TnC (sTnC) and a site I- and II-inactive skeletal TnC mutant (xxsTnC). Reconstituted maximal Ca2+-activated force (rFmax) decreased as the fraction of sTnC in sTnC: xxsTnC mixtures was reduced, but maximal ktr was unaffected until rFmax was 0.2 Fmax) was little effected, though ktr was slightly elevated at low Ca2+ activation. The magnitude of the Ca2+-dependent increase in ktr was greatly reduced following cTnC or xsTnC reconstitution because ktr at low levels of Ca2+ was elevated and maximal ktr was reduced. Solution Ca2+ dissociation rates (koff) from whole Tn complexes containing sTnC (26 ± 0.1 s−1), cTnC (38 ± 0.9 s−1) and xsTnC (50 ± 1.2 s−1) correlated with ktr at low Ca2+ levels and were inversely related to rFmax. At low Ca2+ activation, ktr was similarly elevated in cTnC-reconstituted fibres with ATP or when cross-bridge cycling rate was increased with 2-deoxy-ATP. Our results and model simulations indicate little or no requirement for cooperative interactions between thin-filament regulatory units in modulating ktr at any [Ca2+] and suggest Ca2+ activation properties of individual troponin complexes may influence the apparent rate constant of cross-bridge detachment.